Many techniques exist for treating abnormal cardiac rhythms ("arrhythmias") using cardiac rhythm management systems. For example, too-slow heart rhythms ("bradyarrhythmias" or "bradycardias") are readily treated by external or implantable pacemakers. Such pacers deliver pacing pulses to the heart to evoke a resulting electrical depolarization and accompanying heart contraction. By timing the delivery of pacing pulses, a patient's heart rhythm can be managed. In another example, too-fast heart rhythms ("tachycardias" or "tachyarrhythmias," including "fibrillation") are treated by external or implantable cardioverter/defibrillators (ICDs). Such ICD devices deliver timed pacing pulses to the heart to stabilize its rhythm or alternatively deliver an electrical countershock to interrupt fast electrical conduction paths causing the tachyarrhythmia.
Such cardiac rhythm management systems typically sense intrinsic heart activity signals that are produced by the heart itself Such intrinsic heart activity signals include the electrical depolarizations that cause heart contractions. These signals can be observed using surface electrocardiogram (ECG) equipment (i.e., using external electrodes for sensing intrinsic heart activity) or endocardial electrogram equipment (i.e., using electrodes disposed in the heart for sensing intrinsic heart activity). The cardiac rhythm management system typically bases delivery of therapy (e.g., pacing pulses or defibrillation countershocks) on particular heart rhythms appearing in the intrinsic heart activity signal.
Sensing intrinsic heart activity signals typically involves using a sense amplifier that is coupled to the heart via electrodes. For example, in an implantable pacemaker, an endocardial lead is transvenously introduced into the heart. The lead includes electrodes that are used for both sensing intrinsic heart activity signals and delivering pacing pulses. One known problem with using the same electrodes for both sensing and pacing is the buildup of residual electrical charge on the electrodes as a result of delivering the pacing pulse. Some of the residual charge may be removed by following the pacing pulse with an opposite polarity recharge pulse. Some residual charge, however, typically still exists even after the recharge pulse is delivered. The charge on the electrodes during the pacing and recharge pulses can overload ("saturate") the sense amplifier used for detecting intrinsic heart activity. The sense amplifier is not capable of detecting the intrinsic heart activity signal when the sense amplifier is in its saturated condition. Sense amplifiers may also unnecessarily consume more power when in a saturated condition.
In order to prevent the pacing pulse and accompanying residual charge from saturating the sense amplifier, the sense amplifier is typically "blanked," (i.e., decoupled from the electrodes by switches during the pacing pulses and during recharge time periods). The sense amplifier is reconnected to the electrodes shortly after the recharge pulse is delivered. Even using blanking techniques, several problems still exist. First, there remains some residual charge on the electrodes even when the sense amplifier is reconnected to the electrodes. This may cause a switch closure transient voltage on the heart activity signal sensed by the sense amplifier. Second, the sense amplifier is unable to provide information from the heart during the blanking time periods when it is disconnected. Losing information from the heart during blanking periods is particularly disadvantageous when managing fast cardiac rhythms (e.g., atrial flutter) because, for faster rhythms, more information is lost. Third, blanking techniques require additional components and control circuits, adding cost and complexity to the cardiac rhythm management system. There is a need for improved techniques for sensing heart activity and delivering pacing therapy to a patient.